feat(developer): Allow customisation of modifier keys in OSK

Need to localize modifier keys (Shift, Ctrl etc) < from Daniel Yacob. In further discussion, what he is looking for is ability to add sub-captions to modifier keys, not necessarily change text of modifiers themselves. For example, a hint on RAlt that it gives access to symbols. Part of a larger desire for a dynamic OSK (which I've designed several times in the past and abandoned each time as unwieldy in practice).